Optical modulators by means of which a light beam may be modulated are, among other things, employed in optical telecommunications. The light beam may be transmitted through free space or through a light guide, such as, for example, a glass fiber. Electroabsorption modulators are optical modulators which may be used for modulating the intensity of a laser beam by means of an electric voltage. The electric field within the electroabsorption modulator causes a change in the absorption spectrum, which in turn changes the band gap energy (and thus the photon energy of an absorption edge).
An electroabsorption modulator may be considered to be an electro-optical transducer which converts an electric signal to a corresponding modulation of a light beam or laser beam. The electric signal may, in particular, be a radio-frequency (RF) signal which is transmitted as an input signal to the electroabsorption modulator via a suitable line and a suitable terminal. Since in most cases the impedance of the electroabsorption modulator is not identical to the characteristic impedance of the line, a terminating resistor or terminating impedance may be provided which is mostly electrically connected in parallel to the electroabsorption modulator in order for the overall impedance of the corresponding parallel connection of the electroabsorption modulator and the terminating impedance circuit to be similar to the characteristic impedance of the line. In this way, reflections at the interface between the line and the electroabsorption modulator may be largely avoided.
Some approaches for wiring electroabsorption modulators (EAMs) make use of several and discretely realized electric components (such as, e.g., ohmic resistors and idealized delay line (L, C portions). Usually a hybrid setup is described and used in a technological realization. The term “hybrid setup” here refers to the spatially closely neighboring arrangement of a semiconductor chip and the EAM and a dielectric substrate with a line arrangement and, maybe, further passive components.
When realizing a radio-frequency circuit comprising discrete electric components, typically the impedance values of a circuit may be only of a limited precision, except the circuit is tuned to the desired impedance value in a manual and complicated manner. One possible reason for the limited precision may be that the electric connections (such as, e.g., solder connections) between the discrete components and the circuit board and the bond wire connections may be subject to relatively large variations with regard to their impedance values.
Furthermore, in particular in discrete ohmic resistors, the electric power is implemented within a relatively small space. This may result in a local concentrated heating of the circuit, i.e. in direct proximity to the discrete ohmic resistor. The heat generated by the ohmic resistor has to be dissipated on the one hand and, on the other hand, the corresponding change in temperature in turn may influence the impedance values of the discrete components, which may result in an additional variation of the impedance values of the entire circuit.
Consequently, the object underlying the present invention is providing a terminating impedance circuit which exhibits high precision and stability with regard to its impedance values. Another aspect of the invention is realizing the electric circuit described while avoiding an additional thermal stress for the EML (electroabsorption modulator integrated laser) despite the spatial proximity to the EAM, which is desirable from an RF-technological point of view. This may exemplarily be achieved by using a suitable dielectric in combination with the meandering implementation of the electric line. However, without such thermal decoupling, such an integration of the EML and the terminating impedance circuit would frequently not be practical.